This invention relates to catalysts and methods for removing nitrogen oxides and hydrocarbons from exhaust gas from small utility engines powered by gasoline or gasoline which has been reformulated by the addition of ether or alcohol (hereinafter referred to as "gasoline-fueled"). More specifically, this invention relates to methods for simultaneous removal of hydrocarbons (HC) and nitrogen oxides (NO.sub.x) from the exhaust gas of utility engines smaller than about 25 horsepower, with essentially no conversion of CO contained in such gas (i.e., less than about 3% of the CO contained in the exhaust converted to CO.sub.2).
"Small utility engines" are generally accepted as being defined by the characteristics used by the CARB (California Air Resources Board) in setting regulatory standards for engine exhaust emissions. Those characteristics recite that "small utility engines" are two-stroke and four-stroke, air or liquid-cooled, gasoline, diesel and alternate fuel powered engines under 25 horsepower (18.6 kW) for powering lawn, garden and turf maintenance implements and timber operations equipment; for generating electricity; and for pumping fluids. They are typically designed to be used in, but not limited to use in, the following applications: walk-behind mowers, riding mowers/lawn tractors, garden tractors, snow blowers, edge trimmers, string trimmers, blowers, vacuums, tillers, chain saws, pumps, and other like miscellaneous applications. See generally, Title 13, Cal. Code of Regulations, Sections 2400-2407 (1995). Those characteristics are intended to be used in defining the term "small utility engines" in connection with this invention.
Regulations are being considered or have already been enacted to limit the exhaust emissions from small utility engines. One of the most promising ways for engine manufacturers to comply with current and future emission standards for small utility engines is through the use of catalysts. Three-way catalysts (catalysts which simultaneously oxidize CO and HC while reducing NO.sub.x) are widely used to purify toxic emissions of automotive (i.e., car and truck) internal combustion engines. Small utility engines, however, present an environment with a number of challenges for emission catalyst activity and durability which are not found with automotive exhaust. For example small utility engines typically will provide extremely short catalyst residence times, high hydrocarbon and carbon monoxide to oxygen ratios, overall high levels of emissions leading to high reaction exotherms, perturbated flow due to single cylinder operation, and engine mechanical vibrations.
The differences between automotive and small utility engines pose a set of unique challenges in adapting catalyst technology to this application. In relation to automotive engines, small utility engines tend to have pollutant-rich exhaust, size constraints, extremely high space velocity, high temperature exotherms, a need for improved catalytic and mechanical durability, and cost constraints. The engines for lawn and garden equipment are obviously smaller and less expensive than those found in automobiles, but also have a considerably shorter life expectancy (50 to 250 hrs vs over 3000 hrs for automobile engines). Their duty cycles are also quite different.
Small utility engines are generally made to operate at considerably richer air/fuel ratios than automotive engines in order to prevent overheating of the engine. For this reason and to maintain performance, air/fuel ratios of 11 to 13 are the norm. Moreover, large numbers of small utility engines in use today are made with a side valve combustion chamber and thus tend to have greater internal crevice volumes. These two factors contribute to significantly higher concentrations of hydrocarbons in emissions from such engines than is the case for automotive engines, which are ordinarily designed to operate at or about the stoichiometric point, i.e., an air/fuel ratio of 14.55.
By the nature of their chemistry, catalytic oxidation reactions are exothermic, with the temperature increase across the catalyst bed being directly proportional to the amount of pollutant converted. Because of the high level of pollutants in the exhaust from small utility engines relative to that from automotive engines, the temperature increase generated across a catalytic bed used to treat small utility engine exhaust is much higher than that seen across an automotive converter.
With small utility engines there is also insufficient space available to house and mount the emissions control catalyst in the same manner as is used for automobiles and trucks. The separate canister used for an automotive system is generally not a viable option for a small utility engine. In many cases it has been necessary to house the catalytic element for a small utility engine within the muffler, thereby limiting the size of the catalytic unit to that which can fit within the compact muffler unit. As a result of these limitations, high space velocities across the catalytic element are inherent with small utility engines. Space velocities that are orders of magnitude higher than those encountered by automotive catalytic converters are the norm and thus further serve to limit conversion efficiency. Whereas the space velocity for the catalytic converter of an automotive engine is typically in the order of 40,000 V/H/V and would not exceed 100,000, the space velocity through the catalytic element of a small utility engine contemplated by the present invention is generally about 200,000 at engine idle and much higher (even as high as a million) when the engine is under load.
Clearly, purification of exhausts from small utility engines presents a challenge. Several inventors have proposed methods for addressing the need. Overington et al (U.S. Pat. No. 4,903,482) teaches an exhaust system for 2-stroke engines which includes two exhaust flow paths in parallel, the first of which includes a reduction catalyst and the second of which bypasses the reduction catalyst, the downstream ends of the two flow paths being connected together upstream of an oxidation catalyst.
In similar manner, Schlunke et al (U.S. Pat. No. 4,938,178) teaches an exhaust system for 2-stroke engines which employs separate reducing and oxidizing catalyst portions to treat different portions of the exhaust gas during each exhaust period.
Another approach is taught by Lear et al (U.S. Pat. No. 4,924,820). Lear '820 teaches the use of a plurality of exhaust ports and catalyst chambers, each exhaust port from the engine having a catalyst chamber close-coupled thereon. Lear '820 teaches that the catalyst chamber is effective in decomposing NO.sub.x and in oxidizing hydrocarbons, but employs a mixture of active materials as catalysts.
As is evident from the discussion above, other inventors have relied upon a plurality of catalyst chambers using a plurality of catalysts to reduce harmful emissions from small engines. Discovery of catalysts that would permit the use of one catalyst in a single chamber to reduce emissions from small utility engines, therefore, would clearly be a significant improvement in the state of the art.
Many researchers have taught the use of one catalyst in a single chamber to treat exhaust gas from automotive engines. For instance, Unland (U.S. Pat. No. 3,886,260) teaches the use of a rhodium on alumina catalyst to reduce simultaneously the amounts of nitrogen oxides, hydrocarbons and carbon monoxide contained in exhaust gases from automotive internal combustion engines. McCabe et al (U.S. Pat. No. 5,597,772) teaches a method of preparation of a rhodium on .alpha.-alumina catalyst useful as a three-way automotive emissions control catalyst. These teachings, however, relate to automotive engines which do not present the same challenge as is presented by small utility engines.